202 Edivm G. Conklin. 



duced from any portion of an egg which does not include the whole 

 of the right or left hali. In my opinion Driesch and Crampton 

 have not studied nor taken any account of anterior or posterior 

 half embryos, but only of right or left ones. The question 

 whether these embryos were actually complete will be considered 

 when we come to deal with the various larval organs. 



Both Driesch and Crampton make the claim that single blasto- 

 meres of the 4-cell stage of the ascidian egg may give rise to entire 

 larvse. This is a crucial test of their views, for while it is possible 

 and I believe practically certain that all their "complete larvae of 

 half size" were derived from the right or left halves of the egg and 

 so included portions of all the various ooplasmic substances, this 

 explanation could not apply to their quarter embryos. Driesch 

 figures a larva with all the principal organs (his Fig. 16), which 

 he says is derived from one of the first four blastomeres. How- 

 ever, in size it is as large as any of the half larvae which he figures, 

 and I have no doubt that it is such. 



Crampton figures correctly the early cleavages of one of the 

 anterior quadrants and he gives two figures of quarter larvae, 

 probably of an advanced stage; these figures, however, show no 

 structure whatever save that there is an outer layer around the 

 embryo. There is absolutely no evidence that these embryos are 

 complete. Crampton calls attention to the fact that the long axes 

 of these quarter embryos "are approximately parallel to the 

 principal dorso-ventral axis of the original egg," a fact which I 

 also can confirm. (See my Figs. 66, 69, 70.) He does not, however, 

 determine the fact, which he apparently assumes, that the long 

 axes of these quarter embryos correspond to the long axis of a 

 normal embryo. This is actually not true, as I have shown; the 

 long axes of the quarter embryos are not antero-posterior in 

 direction but dorso-ventral and there has not therefore been any 

 shifting of the axes nor of the ooplasmic substances of these 

 quarter embryos. 



Whether a larva derived from the right or left half of the eo-p; is 

 complete or not can be determined only by a study of the various 

 systems of larval organs. It is evident that parts of all organs 

 which are normally formed along the median plane (first cleavage 

 plane) would appear in an embryo derived from one of the first 

 two cleavage cells, even if the development were strictly partial; 

 the really decisive test as to whether such an embryo is complete 



